U.S. patent number 4,857,737 [Application Number 07/080,015] was granted by the patent office on 1989-08-15 for gamma ray measurement utilizing multiple compton scattering.
This patent grant is currently assigned to Hamamatsu Photonics K. K.. Invention is credited to Ryoji Enomoto, Tuneyoshi Kamae.
United States Patent |
4,857,737 |
Kamae , et al. |
August 15, 1989 |
Gamma ray measurement utilizing multiple compton scattering
Abstract
A detecting unit is formed by disposing 2-dimensional position
sensitive type radiation detectors in the form of a plurality of
layers superposed on each other, each capable of determining
position of each of the reactions in multiple Compton scatterings
produced by a .gamma.-ray in the detecting unit and the energy
which the .gamma.-ray loses there are measured with a high
precision. For each of the reactions it is examined whether the
energy and the momentum conservation laws are satisfied or not in
order to estimate probable sequences of reactions and thus a first
and a second reactions are identified and the direction of the
incident .gamma.-ray is presumed. The measurement precision is
further improved by assigning the energy of the incident
.gamma.-ray presumed from the reactions to the energy of a line
spectrum.
Inventors: |
Kamae; Tuneyoshi (Tokyo,
JP), Enomoto; Ryoji (Sakura, JP) |
Assignee: |
Hamamatsu Photonics K. K.
(Shizuoka, JP)
|
Family
ID: |
26501691 |
Appl.
No.: |
07/080,015 |
Filed: |
July 31, 1987 |
Foreign Application Priority Data
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|
|
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Aug 4, 1986 [JP] |
|
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61-183145 |
Dec 25, 1986 [JP] |
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61-314126 |
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Current U.S.
Class: |
250/370.09;
250/370.1; 250/363.02 |
Current CPC
Class: |
G01T
1/1642 (20130101); G01T 1/2985 (20130101) |
Current International
Class: |
G01T
1/164 (20060101); G01T 1/29 (20060101); G01T
1/00 (20060101); G01T 001/164 () |
Field of
Search: |
;250/327.2C,327.2B,363SB,363SR,367,369,370.09,370.10 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
J Simone, T. O'Neill, O. T. Tumer and A. D. Zych, "Monte Carlo
Simulation of a New Gamma Ray Telescope", IEEE Trans. on Nuclear
Science, vol. NS-32, No. 1, (Feb. 1985), pp. 124-128. .
M. Singh and D. Doria, "Germanium-Scintillation Camera Coincidence
Detection Studies for Imaging Single Photon Emitters", IEEE Trans.
on Nuclear Science, vol. NS-31, No. 1, (Feb. 1984), pp. 594-598.
.
V. Schonfelder et al., "The Imaging Compton Telescope Comptel on
the Gamma Ray Observatory", IEEE Trans. on Nuclear Science, vol.
NS-31, No. 1, (Feb. 1984), pp. 766-770. .
C. A. Carlsson and G. Alm Carlsson, "The Use of the Compton Effect
in Diagnostic Radiology", Proceedings of International School of
Physics, J. R. Greening, ed., pp. 459-473. .
A. C. Damask, C. E. Swenberg, "New Techniques of Brain Studies:
Autoradiography, Positron Annihilation, and Nuclear Magnetic
Resonance", in Medical Physics, vol. 3, Academic Press, Inc.,
(1984), pp. 232-287. .
T. Kamae, R. Enomoto and N. Hanada Galleys, "A New Method to
Measure Energy, Direction, and Polarization of Gamma-Rays", Nuclear
Instruments and Methods in Physics Research, North Holland, (1987).
.
Ronald J. Jaszczak, P. Edward Coleman and Chun Bin Lim, "Spect:
Single Photon Emission Computed Tomography", IEEE Transactions on
Nuclear Science, vol. NS-27, No. 3, (June 1980), pp. 1137-1153.
.
"A New Method to Measure Energy, Direction, and Polarization of
Gamma Rays", T. Kamae, R. Enomoto and N. Hanada, Nuclear
Instruments and Methods in Physics Research, A260, (1987), pp.
254-257. .
"Prototype Design of Multiple Compton Gamma-Ray Camera"-T. Kamae,
N. Hanada and R. Enomoto, Reprinted from IEEE Transactions on
Nuclear Science, vol. 35, No. 1, Feb. 1988..
|
Primary Examiner: Hannaher; Constantine
Attorney, Agent or Firm: Oliff & Berridge
Claims
We claim:
1. A .gamma.-ray measuring device comprising:
a plurality of .gamma.-ray detectors superposed on each other, in
each including means to determine the position of a reaction, when
a .gamma. -ray induces it therein and to measure the energy
imparted to an electron by the .gamma.-ray; and
calculation processing means treating measured data according to a
predetermined program, which sets all the possible sequences of
reactions for a plurality of reactions measured almost
simultaneously, and treats the measured data on said plurality of
reactions while checking for each of the sequences of reactions to
determine whether they satisfy the energy and the momentum
conservation laws, and leaving only the data with an estimated
sequence of reactions which satisfy both the conservation laws.
2. A .gamma.-ray measuring device comprising:
detecting means including 2-dimensional position sensitive type
radiation detectors in the form of a plurality of layers superposed
on each other for detecting the position of a reaction due to a
.gamma.-ray in each of the layers and the energy imparted to an
electron at the position;
an anticoincidence counter having a window in its frontal portion
and enveloping said detecting means for detecting the .gamma.-ray,
which is not absorbed by said detecting means by the photoelectric
effect; and
means for supposing when a value detected by the anticoincidence
counter is smaller than a predetermined value, all the possible
sequences of reactions for a plurality of reactions observed by one
measurement, and checking whether the scattering at each of the
positions of a part or all of the reactions, whose sequence is
supposed, is consistent with the energy and the momentum
conservation laws within predetermined tolerable errors or not.
3. A .gamma.-ray measuring device according to claim 2, further
comprising an auxiliary detector disposed between said detecting
means and said anticoincidence counter, having a window in its
frontal portion and enveloping said detecting means for detecting
the .gamma.-ray, which is not absorbed by said detecting means by
the photoelectric effect.
4. A .gamma.-ray measuring device according to claim 2, wherein
said radiation detectors are semiconductor detectors.
5. A .gamma.-ray measuring device according to claim 3, wherein
said radiation detectors are semiconductor detectors and said
auxiliary detector is a scintillation counter.
6. A .gamma.-ray measuring method utilizing multiple Compton
scattering comprising the following steps of:
disposing 2-dimensional position sensitive type radiation detectors
in the form of a plurality of layers superposed on each other and
detecting the position of a reaction due to an incident .gamma.-ray
at each of the layers and the energy imparted to an electron at the
position; and
supposing all the possible sequences of reactions for a multiple
scattering process, checking whether the scattering at each of the
positions of part or all of the reactions in each of the supposed
possible sequences of reactions is consistent with the energy and
the momentum conservation laws within predetermined tolerable
errors or not, and obtaining analyzed date with estimated sequences
of reactions which are consistent with the conservation laws.
7. A .gamma.-ray measuring method utilizing multiple Compton
scattering according to claim 6, wherein the energy of the incident
.gamma.-ray is calculated by adding all the energies detected in
the detectors where reactions are induced.
8. A .gamma.-ray measuring method utilizing multiple Compton
scattering according to claim 6, wherein the direction of the
incident .gamma.-ray is obtained by using the estimated sequence of
reactions.
9. A .gamma.-ray measuring method utilizing multiple Compton
scattering according to claim 6, wherein the polarization plane or
the polarization factor of the incident .gamma.-ray is obtained on
the basis of the distribution of the Compton scattering
cross-section vs. the azimuthal angle.
10. A gamma ray detector comprising a multi-layered structure of
position-sensitive radiation detector sheets each being capable of
detecting a two dimensional position of a reaction occurred therein
and an energy associated with the reaction, said detector further
comprising an auxiliary detector surrounding said multi-layered
structure except at an entrance window area.
Description
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
This invention relates to .gamma.-ray measurement and in particular
to .gamma.-ray measurement utilizing multiple Compton
scattering.
In this specification .gamma.-ray means high energy photons and
includes soft and hard X-rays. Specifically the object of this
invention is photons having an energy from about 100 keV to about 5
MeV.
Further, a multiple Compton scattering, called hereinbelow
reactions, includes both a phenomenon, by which a photon is
scattered a plurality of times by the Compton effect and a
phenomenon, by which it disappears at last while giving an electron
its energy by the photoelectric effect.
2. DESCRIPTION OF THE RELATED ART
.gamma.-ray is measured in various fields. Taking computer
tomography (CT) as an example, there are roughly known two types,
i.e. positron CT and single gamma CT. In the positron computer
tomography (CT) a radioactive isotope emitting positrons is
introduced in a human body, an animal or a plant and oppositely
directed 2 photons, each of about 511 keV, generated at the pair
annihilation of a positron and an electron are measured by the
coincidence method by means of a number of .gamma.-ray detectors
arranged suitably, such as NaI, CsI, bismuth germanate (Bi.sub.4
Ge.sub.3 O.sub.12, BGO), etc. In this way the spatial distribution
of .gamma.-ray sources, i.e. the isotope, is measured. The location
of a radiation source is confined to a straight line connecting two
detectors, which have detected the two photons by the coincidence
method. Further, the position or distribution of radiation sources
of a same nature is measured 3-dimensionally by detecting a number
of pairs of photons and treating statistically detected data by
means of a computer.
Single gamma CT uses a shield. When a gamma ray is detected, a
gamma ray source is estimated to be located on a line connecting a
detector and a shield. A multiplicity of gamma rays from the same
source are detected and such detected data are statistically
processed in a computer to measure the position or distribution of
gamma ray sources three-dimensionally.
In order to measure the position of incidence of the .gamma.-ray,
an assembly of a number of unit detectors is prepared and it is
sufficient to examine which unit detector the .gamma.-ray enters.
The position of incidence can be detected, e.g. by forming diodes
in a sheet shaped semiconductor substrate, forming a number of
strip-shaped electrodes on the front and backside surfaces thereof,
which cross perpendicularly to each other, and detecting between
which electrodes electric current flows.
In order to know in which direction the .gamma.-ray enters by means
of a measuring device, single Compton method can be employed or
another method can be employed by which a collimator is located in
front of a .gamma.-ray detector so that only .gamma.-ray entering
in a predetermined direction is detected.
A single Compton method is known for relatively low energy
.gamma.-ray, by which the incident direction or the polarization of
the incident .gamma.-ray is measured by using a single Compton
scattering and another reaction. The direction on the polarization
of X-ray coming from e.g. the universe is measured by means of a
measuring device consisting of a position sensitive radiation
detector disposed in the front portion, closer to the object to be
measured, measuring the position of the Compton scattering and the
energy of a recoil electron and an NaI scintillation counter
disposed behind the detector by a suitable distance, which absorbs
the X-ray or the .gamma.-ray after the scattering, giving rise to
scintillation, in order to measure its energy and position.
In the positron computer tomography (positron CT) the energy of the
incident .gamma.-ray is about 510 keV, which is sufficient to give
rise to a plurality of Compton scatterings, i.e. which is an energy
sufficient for every incident .gamma.-ray to produce a plurality of
detection signals within a detecting device.
In single gamma ray tomography, gamma rays ranging from several
hundreds KeV to several MeV are used. Such gamma rays also have
sufficient energy to produce a plurality of Compton scattering in a
detector. The positron CT had the following restrictions.
(a) Since radioactive isotopes usable therefor are limited to
nuclides, which emit positrons and have relatively short
half-lives, it can be utilized only at a location, in the
neighborhood of which such nuclides can be produced.
(b) The positional precision for defining the position of a
radiation source is determined by the positional detecting
precision of the .gamma.-ray detecting device. A several millimeter
square is the lower limit in practice at present even by using a
collimator, etc. The use of the collimator brings about lowering of
the counting efficiency.
(c) Because of the coincidence measurement the counting efficiency
is very low and therefore a measurement takes a long time.
(d) The case where a radioactive isotope emitting positrons is
used, since a positron is emitted with a relatively high energy,
the position, where it annihilates to produce two photons, is apart
from that of the radiation source.
The method for measuring .gamma.-ray the single gamma CT utilizing
by the single Compton scattering method has the following
problems.
(a) In order to increase the counting efficiency, with which a
photon induces a reaction within a detector as expected, it is
necessary to increase the thickness of the sheet-shaped .gamma.-ray
detector, within which it is expected for the photon to induce a
Compton scattering. On the contrary, in order to measure the
positions of two successive reactions with a high precision and to
increase the precision of the measurement of the direction of the
incident .gamma.-ray, it is necessary to reduce the thickness of
the sheet-shaped detector. Since the detection efficiency is
lowered when the thickness of the detector is reduced, the
detection efficiency and the measurement precision are in a
contradictory relation.
(b) When the energy of the .gamma.-ray, which is to be measured, is
above several keV, the number of reactions until a photon is
finally absorbed is increased and the result, it is not possible to
determine the direction of the .gamma.-ray after a scattering,
which is one of the most important factors for determining the
direction of the incident .gamma.-ray, with a high precision and
therefore the measurement precision for the direction of the
radiation and the position of the source is lowered.
(c) In order to increase the detection efficiency, the detector
located behind the examined body should be predominantly thicker
than the detector located in front thereof, within which it is
expected for the first Compton scattering to be induced. In this
case the probability is also increased, that the .gamma.-ray passes
through the front detector without reaction, produces a back
scattering by the Compton effect in the proximity of 180.degree.
within the back detector, and is detected by the front detector. If
the data thus obtained were interpreted, supposing that the first
scattering is produced within the front detector and the second
scattering is induced in the back detector, this gives rise to a
sort of noise, which lowers the reliability of the measurement.
For these reasons the single Compton method is not practical for
the energies above several keV. On the other hand the single gamma
CT method using a collimator lowers extremely the counting
efficiency, because the direction of the radiation is restricted by
the fine collimator (made of e.g. lead). Further it is not
practical, unless the distance between the .gamma.-ray source and
the detector is fixed in a certain extent and stereoscopic
observation is impossible with a single
SUMMARY OF THE INVENTION
An object of this invention is to provide a .gamma.-ray measurement
having a high detection efficiency and a high measurement
precision.
Another object of this invention is to provide a .gamma.-ray
measurement utilizing multiple Compton scattering with a high
spatial precision of the measurement including direction and
position and a high counting efficiency for a .gamma.-ray source
having a line spectrum.
In order to improve the positional precision of the measurement,
thin sheet-shaped detectors are used. Further, in order to increase
the detection efficiency, a plurality of sheet-shaped detectors are
superposed on each other so that a great thickness can be obtained
on the whole. In this way it is possible to measure the position of
each of multiple reactions and the energy of the .gamma.-ray
(energy imparted to electrons). However, in the case where more
than 2 reactions are detected, the direction of the incident
.gamma.-ray cannot be estimated, unless the sequence of the
reactions is not known.
According to one aspect of the .gamma.-ray measurement of this
invention 2-dimensional position sensitive type radiation detectors
are arranged in the form of a plurality of layers superposed on
each other and the position of the reaction in each of the layers,
where the reaction due to the .gamma.-ray is produced, and the
energy imparted to an electron at the position are detected.
Supposing all the possible sequencies of reactions for a plurality
of reactions observed by one meausrement, it is checked whether the
scattering at each of the positions of a part or all of the
reactions, whose sequence is supposed, is consistent with the
energy and the momentum conservation laws within predetermined
tolerable errors or not. In this way a .gamma.-ray measuring method
can be provided, by which possible sequences of reactions are
obtained while excluding the inconsistent sequences of
reactions.
According to another aspect of this invention a .gamma.-ray
measuring device is provided, which comprises a detecting device
consisting of 2-dimensional position sensitive type radiation
detectors disposed in the form of a plurality o layers superposed
on each other for detecting the position of the reaction due to the
.gamma.-ray in each of the layers and the energy imparted to an
electron at the position, an anticoincidence counter having a
window in its frontal portion and enveloping the detecting device
for detecting the .gamma.-ray, which is not absorbed by the
detecting device by the photoelectric effect: and means, which,
when a value detected by the anticoincidence counter is smaller
than a predetermined value, supposing all the possible sequences of
reactions for a plurality of reactions observed by one measurement,
checks whether the scattering at each of the positions of a part or
all of the reactions, whose sequence is supposed, is consistent
with the energy and the momentum conservation laws within
predetermined tolerable errors or not.
According to still another object of this invention a .gamma.-ray
measurement device is provided, which comprises a detecting device
including 2-dimensional position sensitive type radiation detectors
disposed in the form of a plurality of layers superposed on each
other for detecting the position of the reaction due to the
.gamma.-ray in each of the layers and the energy imparted to an
electron at the position; an auxiliary detector having a window in
its frontal portion and enveloping the detecting device for
detecting the .gamma.-ray, which is not absorbed by the detecting
device by the photoelectric effect; an anticoincidence counter
having a window in its frontal portion and enveloping the auxiliary
detector for detecting the .gamma.-ray entering it along a
direction, which does not pass through the window; and means,
which, when a value detected by the anticoincidence counter is
smaller than a predetermined value, supposing all the possible
sequencies of reactions for a plurality of reactions observed by
one measurement, checks whether the scattering at each of the
positions of a part or all of the reactions, whose sequence is
supposed, is consistent with the energy and the momentum
conservation laws within predetermined tolerable errors or not,
thereby to limit or measure the direction of incidence of the gamma
ray.
According to this invention 2-dimensional position sensitive
radiation detectors are arranged in the form of a plurality of
layers superposed on each other and the position of the reaction
due to the incident .gamma.-ray in each of the layers and the
energy imparted to an electron at the position are detected. At
least the energy relative to the position of the first reaction and
the position of the second reaction are obtained while checking
whether the scattering at each of the positions of a part or all of
the reaction, whose sequence is supposed, is consistent with the
energy and the momentum conservation laws within predetermined
tolerable errors or not, supposing all the possible reactions
observed by one measurement. In this way it is possible to measure
the energy and the spatial distribution of the radiation source
with a high precision by measuring a number of photons coming from
the radiation source and treating statistically measurement
results. In this case the radiation source, which is the object of
the measurement, is a .gamma.-ray emitting radioactive isotope
injected or a small amount thereof remaining in a human body, an
animal, a plant, a material, etc. However, this invention may be
applied also to cases where many sorts of radioactive isotopes are
mixedly distributed and thus it is possible to measure
simultaneously the spatial distribution of each of the radioactive
isotopes. Further it may be applied to the measurement of the
energy and the spatial distribution of the X-ray emitted by excited
atoms (or exotic atoms) as by the X-ray fluorescence method.
The present invention is also described in Kamae et al "A New
Method to Measure Energy, Direction and Polarization of Gamma-rays"
in Nuclear Instruments and Methods in Physics Research, North
Holland (1987), which is incorporated herein by reference.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of 2-dimensional position sensitive
type radiation detectors in the form of multiple layers, partially
cut-off, used for realizing this invention;
FIG. 2 is a scheme for explaining the method determining the
incident direction of the X- or .gamma.-ray by means of the
2-dimensional position sensitive type radiation detectors indicated
in FIG. 1;
FIG. 3 is a scheme for explaining the method determining the
3-dimensional position of a radiation source, in the case where 3
photons emitted by a same radiation source are detected;
FIGS. 4A, 4B and 4C are schemes for explaining the .gamma.-ray
detection by the anticoincidence method, FIG. 4A being a
cross-sectional view of the .gamma.-ray detector viewed from the
side, FIG. 4B being a plan view thereof, FIG. 4C being a flow chart
showing the method for determining the incident direction;
FIGS. 5A and 5B show a .gamma.-ray detector having an auxiliary
detector, FIG. 5A being a cross-sectional view thereof viewed from
the side, FIG. 5B being a plan view thereof;
FIG. 6 indicates an energy spectrum obtained by using a measuring
device having the anticoincidence counter;
FIG. 7 is a block diagram of an electric treating circuit used for
the .gamma.-ray measurement according to this invention;
FIGS. 8A and 8B are schemes indicating an analyzing method, in the
case where a plurality of photons are detected, FIG. 8A being an
obtained energy spectrum, FIG. 8B indicating a spatial distribution
of a .gamma.-ray emitting radioactive isotope or fluorescent atoms
emitting a selected line spectrum;
FIG. 9 is a schematical view of a measurement of the spatial
distribution of the radioactive isotope or the fluorescent atoms by
means of a plurality of measuring devices;
FIGS. 10A and 10B are schemes illustrating the dependence of the
Compton scattering cross-section of a polarized .gamma.-ray on the
azimuthal angle .
In the figures the reference numerals represent the following
items; S.sub.1, S.sub.2, . . . , S.sub.n 2-dimensional position
sensitive radiation detectors; P.sub.1, P.sub.2, . . . , P.sub.n
the position of a 1st, a 2nd, . . . , an n-th Compton scattering,
respectively; P.sub.E the position of the absorption of the photon
by the photoelectric effect, G a gap; C a conical surface defined
by the .gamma.-ray; C.sub.1, C.sub.2, C.sub.3 a conical surface
defined by a 1st, a 2nd and a 3rd .gamma.-ray, respectively;
Q.sub.1, Q.sub.2, R.sub.1, R.sub.2, T.sub.1, T.sub.2 the positions
of the 1st and the 2nd Compton scattering of a 1st, a 2nd, a 3rd, .
. ., .gamma.-ray, respectively; 1 2-dimensional position sensitive
type radiation detectors in the form of a plurality of layers
superposed on each other; 2 an anticoincidence counter; 3 an
opening; 4 an auxiliary detector; 5 an anticoincidence counter; 10
2-dimensional position sensitive type radiation detectors;
11.sub.1, 11.sub.2 amplifiers; 12.sub.1, 12.sub.2 waveforming
circuits; 13 a trigger signal generating circuit; 14.sub.1,
14.sub.2 sample hold circuits; 15 a digital circuit; 16 a computer;
17 an auxiliary detector; 20 to 22 .gamma.-ray measuring devices;
and 23 the position of a radiation source.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment will be explained below, referring to figures.
FIG. 1 is a perspective view of a .gamma.-ray detector consisting
of 2-dimensional position sensitive type radiation detectors
superposed on each other in the form of a multi-layered structure;
FIG. 2 is a scheme for explaining the method determining the
incident direction of the .gamma.-ray by means of 2-dimensional
position sensitive type radiation detectors of multi-layered
structure; and FIG. 3 is a scheme for explaining the method
determining the 3-dimensional position of a radiation source, in
the case where 3 photons emitted by a same radiation source are
detected. In the figures, S.sub.1, S.sub.2, . . . , S.sub.n
represent 2-dimensional position sensitive radiation detectors;
P.sub.1, P.sub.2, . . . , P.sub.n the position, where a 1st, a 2nd,
. . . an n-th Compton scattering is produced (detected),
respectively; P.sub.E the position, where the absorption of a
photon is produced (detected), G a gap; C a conical surface
defining the incident direction of the .gamma.-ray, C.sub.1,
C.sub.2 and C.sub.3 in FIG. 3 indicate a conical surface defining
the incident direction of a 1st, a 2nd and a 3rd .gamma.-ray,
respectively and (Q.sub.1, Q.sub.2), (R.sub.1, R.sub.2) and
(T.sub.1, T.sub.2) represent the position of the 1st and the 2nd
Compton scattering of a 1st, a 2nd and a 3rd .gamma.-ray,
respectively.
In the figures, as the 2-dimensional position sensitive type
radiation detector, semiconductor detectors made of high purity
silicon, Li drift type silicon, high purity germanium or high
purity gallium arsenide may be used, which has e.g. an area of
about 10 cm.sup.2 to 500 cm.sup.2 and a thickness of about 200
.mu.m to 5 mm, and more than 2 to about 50 layers of the detectors
are preferably superposed on each other with a gap of about 1 mm to
2 cm. Preferably, all the reactions taking place for each incident
gamma ray is detected. However the size and thickness of each
detector sheet, the magnitude of the gap and the number of layers
are not necessarily restricted to those described above. On the
front and rear surfaces of a semiconductor detector strip-shaped p
and n type electrodes are disposed so as to cross perpendicularly
to each other. Apart from those described above, layers of
proportional chambers filled with rare gas superposed on each
other, drift type solid state detectors, etc. may be used. When a
Compton scattering occurs therein, an electric signal is produced
and the position thereof can be detected through the p and n type
electrodes. Further the energy, which the .gamma.-ray has lost, can
be measured, based on the area of a pulse (current x time).
However, the chronological order of reactions is not directly
detected. Thus, the energy and momentum conservation laws are
utilized to estimate the order. The incident direction of the
incident .gamma.-ray can be specified in a certain extent by using
the direction and the energy of the .gamma.-ray after the
scattering and the condition determined by the energy and the
momentum conservation laws at the scattering.
A measure of the precision for the position measurement required in
the case where the 2-dimensional position sensitive type
semiconductor detectors are used, depends on the thickness of the
semiconductor layers and the gap therebetween. However, when they
are 1 mm thick, it is about 1 mm square. Errors in the position
measurement, when the positional precision is lower than it, and
errors in the measurement of the energy, when the former is higher
than the latter, are principal causes of errors in the distribution
determination of the radiation source of the .gamma.-ray. Noise
produced in the semiconductor decreases with decreasing
temperature. In the case where the semiconductor is germanium, it
is preferable to keep it under about 200.degree. K., and under
150.degree. K., if possible. Also for silicon, in order to improve
the precision of the energy measurement, it is preferable to keep
it under about 200.degree. K. For this purpose heat conductive
(cooling) rods for cooling with liquid nitrogen, a refrigerator,
etc. may be used.
.gamma.-ray passing through a material and having an energy from
about 100 keV to 5 MeV loses its energy by several Compton
scatterings and finally it is absorbed by the photoelectric effect.
By this scattering/absorption electric signals are produced in the
semiconductor. However it is extremely difficult to determine the
sequence of the pulses thus produced. Each of the position
sensitive type radiation detectors S.sub.1 -S.sub.n is so thin that
the probability that more than two Compton scatterings occur within
a same layer, can be neglected. The precision of the position
detection is improved with decreasing thickness of each of the
layers and with more densely superposed layers. Further the number
of all the layers of the detectors is so set that all the energy of
the incident .gamma.-ray is absorbed within the detectors with a
high probability. Unless all the energy is absorbed within the
detectors, it is difficult to estimate the sequence of scatterings
by the theory described later. When the detecting device is so
located that the direction passing through all the layers of the
detecting device thus constructed is directed towards the examined
body emitting the .gamma.-ray, it is possible that the incident
.gamma.-ray undergoes a plurality of Compton scatterings within the
assembly of the detectors with a high probability and that it is
finally absorbed by the photoelectric effect. Therefore the
position, where every reaction occurs, and the energy imparted to
an electron by the reaction, i.e. the magnitude of the pulse,
measured by means of the position sensitive type radiation
detectors S.sub.1 -S.sub.n, are taken-out in the form of electric
signals and recorded in a computer, etc. after having been treated
by means of a suitable electronic circuit. That is, the position,
where the .gamma.-ray undergoes a scattering/absorption, and the
energy, which it loses thereby, are recorded. The recorded
measurement results are treated on-line or off-line according to
the principle, which will be explained below. Now, supposing
that
N: total number of the reaction points within the detecting devices
constructed by 2-dimensional position sensitive type radiation
detectors superposed on each other (total number of hits),
i: number of the reaction point (i =1-N),
E.sub.i : energy imparted to an electron at the i-th reaction
point,
(x.sub.i, y.sub.i, z.sub.i): coordinates of the i-th reaction
point,
E: total sum of E.sub.i (energy of the incident .gamma.-ray).
At first an event for which N >1 is selected. E is calculated
and variations of the count (number of incident .gamma.-rays) are
plotted, the abscissa representing E, the ordinate the count, so as
to obtain a spectrum. A line spectrum, which is the object to be
examined, is selected from the spectrum. Then a window (energy
region), which is wider than the spectrum width observed around the
value of the energy of the line spectrum, is determined, so that
only the events, whose E is within the window, are selected among
all the events, i.e. a .gamma.-ray, which is the object of the
following data processing, is selected. Noise is reduced by the
fact that the object to be measured is a .gamma.-ray having a same
energy.
At first, supposing for each of the .gamma.-rays that the
scattering angle at the i-th scattering be .theta..sub.i, the rest
mass energy of an electron be m, and the energy of the .gamma.-ray
before the i-th scattering be E.sub.ib, the following equation is
valid on the basis of the energy and the momentum conservation
laws; ##EQU1## That is, if the energies before and after the
scattering are known, the scattering angle is known and if the
direction of the .gamma.-ray after the scattering is known, the
probability of the direction of the .gamma.-ray before the
scattering can be known as a point on a conical surface, whose
semi-vertical angle is .theta..sub.i. Then, supposing all the
possible sequences of the reactions, it is checked whether the
scattering at each of the reaction points is consistent with the
equation described above led out from the energy and the momentum
conservation laws within predetermined tolerable errors or not. If
there is a sequence, which is consistent with the above equation,
it is adopted as a correct possible sequence.
For example, in the case where the number of reactions N =4, there
are 4!=4.3.2.1 =24 possible sequences, such as 1 2 3 4, 1 2 4 3, 1
3 2 4, 1 3 4 2, etc. If there is among them a sequence, which is
consistent with the data of 3 Compton scatterings, it is registered
as the correct sequence. If there are a plurality of sequences,
which are consistent therewith, the probability that the
.gamma.-ray follows the relevant sequence in reality is estimated,
and all the possible sequences are registered together with the
weight given to each of the sequences, which is proportional to the
probability. If there is no sequence satisfying the above
condition, the relevant event is avoided. Depending on the
thickness of each of the 2-dimensional position sensitive type
radiation detectors and the total number of the layers, when N is
great, it is possible also to check only a suitable number M (M
<N) of scatterings, e.g. the first 4 or 5 scatterings and to
omit the followings. That is, the check described above is effected
for all the possible M scatterings among N. If there are a
plurality of consistent sequences, they are registered with the
weight attached to each of the events, the total sum of the weights
being 1.
In this way it is possible to estimate correctly the sequence,
according to which the reactions have occured, with a high
probability and to confine the direction of the incident
.gamma.-ray to a conical surface, whose apex is the electric signal
generating point, where it is presumed that the 1st Compton
scattering has occured, and whose rotation axis is the straight
line connecting the two points, where it is presumed that the 1st
and the 2nd Compton scatterings have occured, respectively, as
indicated in FIG. 2. The probability that the sequence according to
which the reactions have occured is presumed correctly, is
remarkably increased by checking each of reactions on the basis of
the energy and the momentum conservation laws. Further, when the
measurement is continued during a certain period of time, a number
of .gamma.-rays coming from a same radiation source are measured.
When the radiation source remains still, the measurement precision
of the energy and the direction is increased proportionally to the
square root of the number of the counts. As indicated in FIG. 3,
obtaining the 3-dimensional distribution of the radiation source,
it is possible to know a stereoscopic image of the radiation source
as intersections of a number of conical surfaces.
On the .gamma.-ray measurement utilizing multiple Compton
scattering described above it is premised that all the energy of
the .gamma.-ray is absorbed within the 2-dimensional position
sensitive type radiation detectors by the Compton scattering and
the photoelectric effect and that it is recorded. For example, for
a plurality of reactions measured almost simultaneously,
.SIGMA.E.sub.i is obtained and the sum of the energies is judged to
be the energy of the incident .gamma.-ray.
In practice, since both the integrated value of the thickness of
the radiation detectors (product of the number of layers and the
thickness of each of the layers) and the area of each of the layers
are finite, a probability that a part of the total energy of the
incident .gamma.-ray goes out of the measuring device remains. The
energy of the incident .gamma.-ray cannot be obtained, even if
.SIGMA.E.sub.i is obtained, for the .gamma.-ray, which undergoes
e.g. 4 reactions within the measuring device and finally goes out
thereof. In this way, even when only a small part of the total
energy goes out thereof, the energy spectrum of the .gamma.-ray
cannot be measured correctly. Further, because of shortage of the
data also the direction cannot be measured correctly. Therefore it
produces noises for the measurement of the energy spectrum and the
distribution of the radiation source, which is the object of the
measurement.
In order to minimize such noises, the anticoincidence method can be
adopted.
FIGS. 4A, 4B and 4C are schemes for explaining a .gamma.-ray
detecting device utilizing the anticoincidence method, FIG. 4A
being a cross-sectional view of the device viewed from the side;
FIG. 4B being a plan view thereof FIG. 4C being a flowchart. In
FIGS. 4A and 4B reference numeral 1 is a detecting unit including
2-dimensional position sensitive type radiation detectors in the
form of a plurality of layers superposed on each other; 2 is an
anticoincidence counter, and 3 is an opening.
The anticoincidence counter 2 surrounds the assembly 1 of the
multi-layered radiation detectors so as to cover it totally except
for the opening 3, through which the .gamma.-ray enters. When only
slight energy is detected by this anticoincidence counter 2, all
the signals due to the .gamma.-ray detected simultaneously by the
multi-layered radiation detectors are avoided. By this method even
a measuring device consisting of a relatively small number of
radiation detectors of small area can select and record only the
cases where all the energy is measured and it is possible to reduce
noise for a relevant line spectrum of the .gamma.-ray.
FIG. 4C indicates a flowchart for determining incident direction of
the Y-ray by using the anti-coincidence counter indicated in FIGS..
4A and 4B.
In the figure an event is selected, for which the total number of
hits N within the 2-dimensional position sensitive radiation
detectors is greater than 1 and the total sum of the energies
imparted to electrons within the anticoincidence counter 2 is
smaller than a predetermined value (Step (i)). The cases are
classified, depending on N and it is checked whether the Compton
scattering satisfies the energy and the momentum conservation laws
for all the possible N! sequences (Step (ii) and (iii)). If there
is a consistent sequence (Step (iv)), as indicated in FIG. 2, the
conical surface is determined and registered by using the first
Compton scattering and if there are a plurality of consistent
sequences, they are weighted (sum of weights =1) and the conical
surface for each of them is registered (Step (v)). As stated
previously, when N exceeds 4 or 5, if there is a sequence, for
which a first suitable number M (M =4 or 5) of Compton scatterings
are consistent, the calculation may be closed there. However all
the possible M scatterings should be checked among N scattering,
(N!/(N-M)!) times.
By the device indicated in FIG. 4, among the incident radiations,
those which have run away to the anticoincidence counter are
avoided. In order to measure these radiations, the measuring device
may be constructed as indicated in FIG. 5.
FIGS. 5A and 5B show another embodiment for detecting the total
energy, FIG. 5A being a cross-sectional view of a detecting device
viewed from the side, FIG. 5B being a plan view thereof, where
reference numeral 4 represents an auxiliary detector and 5
indicates an anticoincidence counter.
In the figures the auxiliary detector 4 surrounds the assembly 1 of
the multi-layered radiation detectors so as to cover it totally
except for the opening 3, through which the .gamma.-ray enters.
Further the anticoincidence counter 5 surrounds them so as to cover
them totally except for the opening 3. In this case the energy
detected by the auxiliary detector 4 is added to the energies of
the .gamma.-ray detected simultaneously by the multi-layered
radiation detectors so as to obtain the total energy of the
incident .gamma.-ray. Furthermore, when only slight energy is
detected by the anticoincidence counter 5, it is recognized as
external noise coming from the exterior, which does not pass
through the opening and the signals induced by the .gamma.-ray
detected simultaneously by the multi-layered radiation detectors
are avoided. The total energy of the .gamma.-ray can be measured
correctly by the fact that it is possible to calculate the total
energy by adding also that detected by the auxiliary detector, and
it is possible to obtain the sequence of reactions within the
2-dimensional position sensitive radiation detectors, to obtain
correctly a sequence of reactions of at least the first and the
second scatterings. In this way, even ith a simple measuring device
consisting of only a small number of radiation detectors of small
area, it is possible to measure the total energy, which permits to
effect measurement by the multiple Compton scattering method with a
smaller detector unit.
The embodiment indicated in FIGS. 5A and 5B using the auxiliary
detector or side counter differs from that indicated in FIG. 4C
only in that in the treatment of measured data the energy detected
by the auxiliary detector is included in the total energy and in
the other points the former is identical to the latter.
FIG. 6 shows an energy spectrum obtained by means of a measuring
device having an anticoincidence counter, from which the cases
where an energy above 50 keV is detected by the anticoincidence
counter are excluded, in which D.sub.1 is a line spectrum of 0.511
MeV; D.sub.2 is an energy spectrum, from which the cases where an
energy above 50 keV is detected by the anticoincidence counter are
excluded; and D3 is an energy spectrum before the exclusion.
From the figure it can be seen that noise is reduced by using the
anticoincidence counter.
As the auxiliary counter and the anticoincidence counter,
semiconductor detectors or scintillation counters made of CsI, NaI,
bismuth germanate (BGO), etc. can be used. In order to detect the
.gamma.-ray from about 100 keV to about 5 MeV without losing it
partially, a thickness greater than about 2 cm is necessary. Both
the auxiliary detector and the anticoincidence counter should cover
totally the 2-dimensional position sensitive radiation detectors
without any gap except for the opening, through which the
.gamma.-ray enters, a passage of conductors for taking out electric
signals, a passage of cooling rods (not shown in the figure), etc.
For this purpose it is easier to fabricate them in an assembly of a
plurality of semiconductor detectors or scintillation counters than
in one body. Furthermore, in order to take out electric signals,
photomultiplies, avalanche type photodiodes, PIN type photodiodes,
etc. may be used.
FIG. 7 indicates the block diagram of an electric signal processing
circuit for the .gamma.-ray measurement, which is an embodiment of
this invention, in which reference numeral 10 is a 2-dimensional
position sensitive radiation detector; 11.sub.1, 11.sub.2 are
amplifiers; 12.sub.1, 12.sub.2 are waveform shaping circuits; 13 is
a trigger signal generating circuit; 14.sub.1, 14.sub.2 are sample
hold circuits; 15 is a digital circuit; 16 is a computer; and 17 is
an auxiliary detector.
In the figure, a signal obtained by the 2-dimensional position
sensitive radiation detectors 10 is amplified and wave-formed,
which gives rise to a trigger signal at the trigger signal
generating circuit 13. This trigger signal serves as a gate signal
for sampling the detected signal and A/D converted data are sent to
the computer, where they are stored and treated. In the case where
the auxiliary detector is used, the signal obtained by the detector
17 is similarly treated, as indicated by broken lines.
FIGS. 8A and 8B show an example of the analysis in the case where a
number of .gamma.-rays are observed. If a part of the energy
spectrum is selected, as indicated in FIG. 8A it is possible to
know the spatial distribution of the corresponding radioactive
isotope or fluorescent atoms with a high precision. It is also
possible to know also the spatial distribution, variations with
respect to time, etc. for every kind of the radioactive isotope or
the fluorescent atoms (energy spectrum).
FIG. 9 is a scheme showing an example of the measurement of the
spatial distribution of the .gamma.-ray emitting radioactive
isotope or the fluorescent atoms by means of 3 .gamma.-ray
measuring devices, in which reference numerals 20, 21 and 22 are
the .gamma.-ray measuring devices, source.
As indicated in the figure not only the statistical precision is
improved, but also the angle of the stereoscopic view is increased
and the precision of the 3-dimensional measured position is
considerably improved by obtaining intersections of conical
surfaces determined by each of the measuring devices by means of a
plurality of .gamma.-ray measuring devices according to this
invention.
In this case also the measurement precision can be improved by
means of a plurality of .gamma.-ray measuring devices, each of
which is provided with an auxiliary detector.
FIGS. 10A and 10B show the dependence of the Compton scattering
cross-section of polarized .gamma.-ray on the azimuthal angle, in
which E indicates the direction of the electric vector of the
incident .gamma.-ray, .phi. the azimuthal angle comprised between
the direction of the electric field of the incident .gamma.-ray and
the scattered direction and .theta. the polar angle of the
scattering.
In the figure the incident .gamma.-ray is 100% polarized at 0.511
MeV. The polarization factor can be known from the difference
between the peak and the valley in a distribution curve and further
the polarization plane can be obtained by knowing in which
azimuthal angle the valley is.
Thus, if the energy of each of incident .gamma.-rays is detected
e.g. by means of 2-dimensional position sensitive type radiation
detectors alone or in combination with an auxiliary detector, it is
possible to effect measurements with a high precision (about 2 keV
to about 10 keV), which is proper to the semiconductor detector, to
confine the incident direction to a conical surface with a high
precision (e.g. from about 1.degree. to about 3.degree.) and to
measure the energy and the polarization plane simultaneously. It is
also possible to obtain a resolving power smaller than 1 keV for
the center energy of a line spectrum and a positional resolving
power of 1 mm cube for the spatial coordinates by repeating the
measurement during a certain period of time in order to accumulate
data and by using a plurality of measuring devices, if
necessary.
Even in the case where only the auxiliary detector made of NaI,
CsI, etc. is used, only the precision in energy is worsened by a
factor of about 10 and the incident direction and the polarization
can be measured with a similar precision. Therefore it is possible
to obtain a similar spatial resolving power.
Further, since it is possible to use any nuclide, if the
.gamma.-ray source has a line spectrum, it is possible to select a
nuclide, which is easily taken-in in a specified organ of a human
body or an animal, or a plant, a nuclide having a relatively long
life, or a nuclide easily produced. Therefore it is possible to
enlarge remarkably the application field. Furthermore, since a
single .gamma.-ray is measured it has a high counting efficiency
and since the device is compact, it is possible to improve the
positional measurement precision and to utilize a large
stereoscopic angle by locating it closely to the object to be
measured therefore measurements can be effected even with a weak
radiation source. For this reason the distribution of the radiation
source and variations in the intensity with respect to time can be
also measured.
* * * * *